Inactivating mutations of PHEX, which underlie XLH, modulate expression of the disease, in part, by regulating serum FGF-23 levels. Our recent studies have determined that the physiologically relevant site for expression of the PHEX mutation is the osteoblast. However, the mechanism whereby inactivation of PHEX alters FGF-23 degradation and/or production and consequently serum hormone levels remains unknown. Nevertheless, our advances have provided the framework to understand this regulatory process. These advances indicate that loss of function PHEX mutations limit production of the chaperone protein, 7B2, in osteoblasts, reducing SPC2[unreadable]7B2 activity. Limitation of this enzyme function results directly in impaired FGF- 23 degradation and through down-stream effects increased FGF-23 production. In related studies, we have used animals with transgenic over-expression of Npt2[unreadable] to document a role for P transport in regulation of renal 25(OH)D-1&#945;-hydroxylase activity in Hyp-mice. In the current application, we propose to use animal models to investigate factors underlying the characteristic phenotypic abnormalities in the Hyp-mouse. Using animals with transgenic over-expression of 7B2 in osteoblasts, we will confirm the central role of the chaperone protein in regulation of serum Fgf-23 in Hyp-mice. In addition, we will establish with certainty that the renal phenotype, of XLH, is exclusively Fgf-23 dependent. As a complement to these studies, we will explore if Fgf-23 likewise regulates bone and cartilage mineralization in Hyp-mice. The model systems employed will permit us to assess if Fgf-23 alone regulates bone mineralization or the effects of the hormone are dependent, in part, on the serum P concentration. Alternatively, the studies may indicate that MEPE-ASARM peptides share with Fgf-23 the regulation of bone mineralization. Experiments to confirm this possibility will include generation of additional animal models, including those derived from crossing the transgenic 7B2-Hyp-mice with Mepe null mice. An extension of these investigations will include experiments to determine if renal P transport, an apparent modulator of renal 25(OH)D-1&#945;-hydroxylase activity in Hyp-mice, universally regulates 1,25(OH)2D production. We will use conditional knockout of Npt2 to test the effect of renal P transport on enzyme function in normal animals. These studies will provide essential information to understand the effects of Fgf-23 on vitamin D metabolism in Hyp-mice. Our studies are significant, as they will clarify dependence of the HYP phenotype on FGF-23 effects, while enhancing our understanding of vitamin D metabolism and P homeostasis.